Neurotrophins are a family of growth factors that help to regulate the survival and differentiation of neurons. They influence cellular behavior through their interaction with two distinct receptors, the Trk tyrosine kinase family of receptors and the low affinity p75NTR receptor. The latter receptor can signal independently of the Trk family, regulating cell death via the generation of ceramide. In addition, it appears that p75 can increase the activity of Trk, and in contrast, that Trk can silence p75 signaling; that is, there seems to be reciprocal interactions between these receptors. There is evidence that these signaling events are initiated in caveolae and caveolae related domains (CRD), lipid-rich domains enriched in cholesterol, glycolipids and specific proteins, including the key protein component caveolin. The general hypothesis to be tested is that "compartmentalization of neurotrophin receptors within caveolae/CRDs and the interaction of these receptors with structural proteins in these domains is critical for the regulation neurotrophin signaling." To test this hypothesis, three specific aims are proposed: In the first aim, experiments are proposed to determine how structural proteins of CRDs interact with Trk receptors and regulate signaling. The investigators will ask if a putative binding domain in Trk receptors in fact regulate interactions with caveolin; if so-called scaffolding domains in caveolin regulates interactions between Trk and p75 receptors; if other structural proteins present in CRDs interact with caveolin, in particular, flottlin; and ask about the mechanism of inhibition of tyrosine activation by caveolin. In the second aim, investigations will be made regarding the molecular mechanism of Trk inhibition by p75 signaling in CRDs. In particular, the mechanism of a ligand-activated sphingomyelinase localized in caveolae will be investigated by determining if Trk activation of the PI3K/PKB (Akt) pathway, and phosphorylation of certain sequences in acid sphingomyelinase, regulate acid sphingomyelinase activity. In aim three, the effect of the lipid composition in regulating the partitioning and signaling of neurotrophin receptors in CRDs will be assessed. These studies will determine if compartmentalization of the receptors into CRDs is in fact necessary, and if the CRD content of cholesterol and sphingomyelin affects the localization of Trk to CRDs, and compromises signaling.